Apparatus for electron beam control



p 3, 1969 A. D. COLVIN 3,469,139

APPARATUS FOR ELECTRON BEAM CONTROL Filed Feb. 27, 1968 A L N l V/ A u u EL uh u/ H Iv/71 1 L4 L 1a 4 ADD [JUDGE] U United States Patent US. Cl. 313-74 2 Claims ABSTRACT OF THE DISCLOSURE A method of and apparatus for controlling an electron beam developed within the vacuum chamber of a particle accelerator. Means are provided for scanning the beam through a plurality of angularly diverging paths about a central axis. A magneticor electrostatic field intersects these paths and deflects the beam such that it passes through the accelerator window in substantially parallel paths. The magnitude of this field varies inversely with the distance from the axis such that the greater the angle of divergence of the path of the beam, the greater the magnitude of the beam deflecting forces caused by the field.

BACKGROUND OF THE INVENTION Tarticle accelerators conventionally are used to irradiate usefully a target by means of an electron beam developed within an evacuated housing chamber and exiting the housing chamber through an opening in a wall thereof. Chamber vacuum is maintained by sealing this opening with an electron permeable window formed from a thin sheet of metal such as aluminum. The beam is emitted by a cathode positioned at one end of the chamber and is accelerated by and directed to an anode by a large potential difference existing between the cathode and anode. The anode structure may comprise in whole or in part the window.

In order to protect the integrity of the window from electron beam energy, the beam usually is scanned along the window so that it impinges along the window in an infinite number of angularly diverging paths. Also, it is advantageous to prevent displacement of the window by forces caused by atmospheric pressure by supporting the window with a grid structure.

Individual elements of the grid must be oriented at angles to one another corresponding to the angular relationship between the electron beam paths in order to allow the beam to pass through the grid and minimize the dissipation of energy caused by the beam impinging on grid elements. Such grids inherently are costly and diflicult to manufacture because of the precise angular dispositions required of the grid elements.

It is an object of this invention to provide a method of and apparatus for controlling the paths of a scanned accelerator electron beam such that the beam passes through the grid and window in parallel paths as it is scanned. The means accomplishing this objective are simple in construction and do not require adjustment during accelerator operation. Such electron beam control allows use of a grid having relatively easily formed, parallel grid elements and prevents excessive and objectionable energy dissipation due to beam impingement on grid structure.

SUMMARY OF THE INVENTION The method of controlling an electron beam so that it is emitted through an electron permeable window of a particle accelerator housing in parallel paths is accomplished by developing in the housing chamber an electron beam directed along an axis intersecting the window. The beam is scanned over the surface of the window. A magnetic field is developed proximate the Patented Sept. 23, 1969 ice window within the chamber. The flux of this field at a point is directly proportional to the distance from this point to the axis. Such a field provides that the scanned beam is deflected through the window in paths parallel to the axis.

A particle accelerator capable of electron beam control according to the objects of this invention includes a housing defining a vacuum chamber. An opening is formed in said housing and an electron permeable window is operatively secured to the housing adjacent the opening and extends at least coextensive with the housing opening. Particle discharge means, e.g. a tungsten wire comprising cathode assembly, are positioned at one end of the chamber and emit a particle beam directed towards the window. Beam deflecting means, e.g. electromagnetic means or electrostatic means, are positioned between the discharge means and the window and are positioned proximate the window and create a beam controlling field such that the paths of the beam through the window during beam scanning substantially are parallel. The window is supported against the forces of atmospheric pressure by a grid operatively secured to the housing and having individual grid elements lying substantially parallel to one another.

DESCRIPTION OF THE DRAWING FIGURE 1 is a schematic elevation view of a particle accelerator including apparatus constructed in accordance with and capable of performing the method of this invention and FIGURE 2 is a sectional view taken along the line 22 of FIGURE 1.

DETAILED DESCRIPTION OF THE INVENTION Referring in detail to the drawing, the numeral 10 denotes generally an evacuated chamber accelerator and includes a tubular member 12 and a rectangular bucket 14 having generally diverging sides. Tubular member 12 and bucket 14 cooperate to define a chamber 16.

A cathode 18, shown greatly enlarged, is positioned at one end of chamber 16 and is capable of emitting a beam of electrons. The open end of bucket 16 remote from cathode 18 has secured thereto a grid 20 including web-like individual grid elements 22. Grid 20 supports an electron permeable window 24 that may be a thin sheet of aluminum or similar material. The closing of chamber 16 by grid 20 and window 24 permits the maintenance of a vacuum within this chamber.

A pair of electromagnetic scanning coils 26 and 28 are positioned on opposite sides of tubular member 12 along the length thereof. As is well-known in the accelerator art, scanning of the electron beam emitted from cathode 18 can be effected by varying the proportion of current passing through coils 26 and 28. For a full discussion of the use of coils similar to coils 26 and 28 for the scanning of electron beams, attention is directed to US. Patent 3,066,238, issued Nov. 27, 1962.

In the end of chamber 16 proximate grid 20 are positioned a pair of magnets 30 and 32. As may be seen from FIGURE 2, window 24 has an oblong shape with each of the magnets 30 and 32 positioned proximate one of the ends of the window. Interconnecting the magnets are a pair of demagnetized iron bars 34 and 36 that lie along the long dimension sides of the grid opening and window.

During operation of accelerator 10, chamber 16 is evacuated and a small diameter beam of electrons is emitted from cathode 18. This beam is directed along the central axis 38 of chamber 16 and is greatly accelerated within chamber 16 due to large potential differences between cathode 18 and the structure at the opposite end of chamber 16 including window 24 that functions as an anode. As the electron enters the magnetic field between and created by scanning coils 26 and 28, it is scanned through an infinite number of angularly diverging paths represented in part by the lines 42. The point of divergence due to the scanning coils 26 and 28 is represented by the point 40. As the electron beam is directed along one of the diverging paths 42, it enters the magnetic field present between the magnets 30 and 32.

Magnets 30 and 32 are of equal magnetic strength and thus it readily may be appreciated that the flux of the field between these magnets varies directly with the distance from a point in the field to central axis 38. Along axis 38 the flux efiectively will be zero as the fields of magnets 30 and 32 will cancel one another. At a point proximate to one of the magnets, however, the effect of the flux from the other of said magnets will be diminished and the greater will be the resultant eflective flux. The greater the angular divergence of a path 42 of the electron beam, the greater will be the magnetic force deflecting the beam. Magnets 30 and 32 are selected such that the field therebetween will deflect the electron beam in any one of the paths 42 such that this path then will be parallel to axis 38. When the electron beam is scanned along the axis 38, no deflection of the beam will occur since the effective field along axis 38 is zero.

It thus may be seen that regardless of the angular divergence of the path 42 through which the electron beam is scanned, the electron beam will pass through grid and window 24 in paths parallel to axis 38. Thus the individual elements 22 of grid 20 may be made parallet to one another as best may be seen from FIGURE 1.

This invention thus provides a method and apparatus for controlling the path of a deflected accelerator electron beam so that the beam passes through the grid and window in parallel paths regardless of the angular displacement of the beam due to deflection. Since magnets 30 and 32 are chosen empirically to compensate for the particular angular divergence of the deflected electron beam, no further adjustment of these magnets need be made. Such an arrangement and control of the electron beam allow the individual grid elements 22 of grid 20 to be formed having a parallel relationship, a relatively simple fabrication operation. Electron energy dissipation due to the impingement of the electron beam against the grid elements is minimized since with the grid elements lying parallel to the path the electron beam takes through the grid structure, the area of the grid elements exposed to the electron beams is minimized.

It is to be understood that this invention is not limited to the exact construction or method illustrated and described above or in the abstract preceding this specification, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

I claim:

1. A particle accelerator comprising: a housing defining a vacuum chamber, an opening formed in said housing, an electron permeable window operatively secured to said housing and at least coextensive with said opening, particle discharge means positioned in said chamber and emitting a particle beam that impinges on said window, beam deflecting means positioned between said discharge means and said window and capable of scanning said beam over at least a portion of the area of said window, and magnetic means positioned proximate said window and creating a beam controlling field such that the paths of said beam through said window during beam scanning are substantially parallel, said window being rectangular, said magnetic means including a pair of permanent magnets, each of said magnets being located proximate one end of said window, and a pair of demagnetized ferro-magnetic members, each of said ferromagnetic members extending between said magnets proximate one side of said window.

2. A particle accelerator comprising: a housing defining a vacuum chamber, an opening formed in said housing, an electron permeable window operatively secured to said housing and at least coextensive with said opening, particle discharge means positioned in said chamber and emitting a particle beam that impinges on said window, beam deflecting means positioned between said discharge means and said window and capable of scanning said beam over at least a portion of the area of said window, and magnetic means positioned proximate said window and creating a beam controlling field such that the paths of said beam through said window during beam' scanning are substantially parallel, said magnetic means including a pair of permanent magnets, each of said magnets positioned proximate opposed sides of said window.

References Cited UNITED STATES PATENTS 1,907,507 5/1933 Coolidge 313-74 2,348,133 5/1944 Iams 315--13 2,373,661 4/1945 De Phillips 3 l3'74 2,977,500 3/1961 Boeker 315-l5 3,066,238 11/1962 Arndt 31522 RODNEY D. BENNETT, JR., Primary Examiner M. F. HUBLER, Assistant Examiner US. Cl. X.R. 3l384 

